Sign clamp



FIG. 1 is a perspective view of a sign clamp showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a right side view thereof;

FIG. 5 is a top view thereof;

FIG. 6 is a left side view thereof;

FIG. 7 is a bottom view thereof. 

The ornamental design for a sign clamp, as shown and described. 